Conventionally, the communication system of the PLC method for performing communication via a power source line has been known. As an example of installing this communication system of the PLC method in a vehicle, a communication system shown in FIG. 11 has been known. As shown in this figure, the conventional communication system 100 includes: a CPU 102 that sends a communication signal according to operation of an operation unit 101; a PLC modulator 103 that superimposes the communication signal from the CPU 102 on a power source line L; a PLC demodulator 104 which demodulates the communication signal that is superimposed on the power source line L; and a CPU 106 which controls a load 105 according to the communication signal that is demodulated by the PLC demodulator 104.
In the conventional communication system 100, the PLC modulator 103 and the PLC demodulator 104 are connected to each other in parallel. Thus, since the PLC modulator 103 superimposes the communication signal on the power source line L on a side closer to a battery 107, which is shown by a bold line in the figure, the communication signal is superimposed not only on the power source line L between the PLC modulator 103 and the PLC demodulator 104, but also on the power source line L that is to be connected to another load 108. Since the power source line L is provided to have significantly low impedance, the communication signal may be attenuated, in the case where the other load 108 connected to the power source line L is a capacitive load.
Then, it would be considered to be effective to separate the other load 108 from the power source line L while performing the PLC communication, but there are problems of limiting a timing for performing the communication and the like.